Project Summary/Abstract Plasmodium vivax Duffy binding protein ligand domain (DBPII) is a leading vivax vaccine candidate based upon its relative importance for parasite survival during the disease-causing blood stage. However, DBPII is naturally weakly immunogenic and induces strain-specific immunity. Our approach to solve the problem strain immunity is to design a vaccine that focuses antibody responses onto conserved B-cell epitopes in functional regions within DBPII that are proven targets of protective immunity. In a proof-of-concept study, we demonstrated that an engineered DBPII immunogen lacking the dominant variant B-cell epitope retained good immunogenicity and induced more broadly inhibitory antibodies (BIAbs). The results for the DBPII- engineered vaccine is promising and the next critical step will be to determine the requirements to induce long-lived antibody and memory B-cell (MBCs) responses targeting conserved neutralizing epitopes. The overall goal of this program is to design a vaccination strategy to induce the distinct subpopulations of B- cells that are expanded in people with protective immunity to vivax malaria. Our hypothesis is people able to produce broadly naturally acquired DBPII BIAbs (elite responders) mount an efficient and long-term DBPII- specific memory B-cells (MBCs) and this can be replicated by vaccination. We will phenotypically and functionally characterize B cell sub-sets in immune individuals, to optimize the design of new DBPII vaccine that can induce MBCs cells with specificity for different DBPII. For that, we will take advantage of Brazilian cross-sectional and cohort studies, where different profiles of DBPII responders were identified, including persistent responders with strain-transcending DBPII-BIABs. DBPII is expected to be an important component of a multi-stage, multi-valent vaccine to protect against vivax malaria. The proposal builds on a solid and successful collaboration established between the Brazilian and US collaborators, combining strengths parasitology, immunology, and structural biology to optimize DBPII as an effective vaccine against vivax malaria.